An early and highly formative entry into what would eventually become heavy metal, this 1971 behemoth of a record is Deep Purple's sixth, and was famously recorded in The Rolling Stones Mobile Studio (a truck). Marked by fat guitars doubled by fatter keyboards, Machine Head's hits -- "Highway Star," "Smoke on the Water" and "Space Truckin'" -- remain among the band's most recognized songs. An interesting fact is that approximately 99.9 percent of the time, "Smoke on the Water" is the very first rock riff beginning guitarists learn.